Single tube drilling assemblies and multi-tube concentric drilling assemblies are both accepted equipment for conventional fluid injection drilling. Additionally, multi-tube concentric drilling assemblies are utilized in reverse circulation drilling. A system for dual tube drilling is disclosed in Henderson, U.S. Pat. No. 3,208,539.
In conventional drilling, utilizing single tube drilling assemblies, drilling fluid (e.g., mud or water) is pumped down the drill pipe to the bit or tool and returns to the surface through the annular space between the drill pipe and the hole wall (the hole annulus). Conventional circulation utilizing multi-tube concentric drilling assemblies allows the fluid to be circulated down either or both the inner tube passage and the pipe annulus (the passage between an outer and inner tube disposed concentrically), across the cutting face of the tool and returned to the surface through the hole annulus. This latter mode may be used to keep two fluids separated until they are at or near the bottom of the hole, where they would be mixed, possibly forming a third substance with different characteristics, such as a stiff foam, gel, liquid or solid having the particular chemical or physical properties needed at or near the point of mixing.
In reverse circulation dual tube drilling systems, the drilling fluid is pumped down to the bit through the pipe annulus to the site of the bit. The fluid returns to the surface through the inner pipe.
It is important to rotary drill systems generally, of which the above described are examples, to provide additional lateral support to the drill stem to limit buckling of such portions which may be subject to compression, or to maintain borehole alignment. Such lateral support can be provided by borehole devices generally known as stabilizers.
The majority of stabilizers in general use are equipped with diameter control blades or wear pads which rotate with the drill stem and are therefore subject to circumferential surface wear by the formation being drilled. Also the blades or wear pads of such tools tend to cut into the walls or under-ream the borehole in which they are employed.
Other types of stabilizers are equipped with non-rotating blades or wear pads which are mounted on a sleeve and do not rotate with respect to the hole wall. These devices have the disadvantage of rapid wear between the non-rotating stabilizer sleeve and the rotating stem or mandrel which wear is also increased by the entry and accumulation of abrasive particles between these surfaces from the circulating fluid as it ascends from the bit or cutting head.